1. Field of the Invention
The present invention relates generally to the field electric motor controls. More particularly, the invention relates to a variable frequency motor drive capable braking and controlling the deceleration of an electric motor.
2. Description of the Prior Art
Since their inception a number of years ago, variable speed, alternating current (AC) motor drives have gained tremendous acceptance for a wide range of industrial applications. These devices now serve as primary drivers for such machines as pumps, conveyors and material handling equipment, replacing other drive systems such as direct current motors and mechanical or electromechanical variable speed drive devices. As the quality and availability of the components of variable speed AC drives has improved, particularly of semiconductor components such as microprocessors, these drives have become even more economical and reliable.
The popularity of variable speed AC drives, and adjustable frequency drives in particular, has been due, in part, to their ease of installation and use. Widely available drives may be installed and connected between a polyphase or single phase power source and a conventional AC motor. Properly programmed, such drives enable an operator to simply select a desired output speed, acceleration rate and deceleration rate, and to limit current and voltage applied to the motor in accordance with specifications supplied by the motor manufacturer. Moreover, available interfaces permit these functions to be easily automated, such as in a computerized manufacturing or process control system wherein the speeds of one or more motors are set and controlled automatically as required by the control system.
Known adjustable frequency AC motor drives are generally designed to control the output speed of polyphase induction motors. Because the synchronous or steady state speed of such motors is dependent upon and directly proportional to the frequency of their input current, their speed may be selectively varied by controlling the supply current frequency. The voltage applied to the motor is generally varied proportionately with the frequency such that their maximum values are reached at the rated voltage and frequency of the motor. In one known technique, the output voltage for each AC phase is controlled by generating a pulse-width-modulated (PWM) voltage waveform corresponding to the output voltage level desired. Moreover, the output current applied to the motor is varied, up to the rated motor current limit, to supply the necessary torque to drive the motor load while maintaining the desired output speed characteristics. In addition to controlling the final motor speed, such drives typically include algorithms for controlling acceleration and deceleration of the motor by increasing or decreasing the driving frequency and voltage at a desired or target rate.
While motor drives of this type have proven extremely useful in driving conventional motors at desired speeds, they do not always provide satisfactory transient characteristics, particularly during deceleration of inertial loads. Where a motor is coupled to a primarily frictional load, i.e. a load that inherently tends to decelerate at a rate greater than the desired rate, the device effectively drives the load constantly such that the load slows at the desired rate. On the other hand, inertial loads, i.e. those that tend to decelerate at a rate below the desired rate, are not adequately slowed and may require braking to slow them to the desired speed at the desired deceleration rate. Such braking may typically be provided by dynamic brake resistors designed to absorb energy generated by the motor during deceleration. However, such techniques add expense and bulk to the drive package. Moreover, other known braking techniques, such as DC braking, while effective at stopping a motor, do not offer satisfactory control of deceleration rate and are not suited to deceleration from one speed to another without ultimately stopping the motor.
The present invention is intended to respond to these shortcomings of known motor drives. In particular, the invention provides an adjustable frequency AC motor drive system capable of decelerating inertial as well as frictional loads at a controlled rate upon demand. Moreover, there is a need for a drive system which affords controlled deceleration in a motor drive that is both competitive with existing drive packages and compatible with conventional AC motors.